How to Dynamically Load External JARs into Spring Boot at Runtime

Implementation Plan

The solution consists of two steps: load the external class into the JVM and let Spring scan it.

Load Class into JVM

Assume the JAR to be loaded is placed in /user/local/java/plugins. Three implementation methods are available:

Expand the -cp of the AppClassLoader to load external JARs.

Use Spring Boot’s loader.path startup parameter.

Define a custom ClassLoader to load JARs from a fixed directory or an environment variable.

Methods 1 and 3 rely on JDK mechanisms; method 2 is a Spring Boot extension.

Solution 1 – Expand -cp

Specify the classpath with the classpath option, but it only works when the JAR is started via its Main method. The Spring Boot manifest shows Main-Class: org.springframework.boot.loader.JarLauncher, so the launch command must be changed to:

java -cp /user/local/java/plugins org.springframework.boot.loader.JarLauncher

Advantage: simple implementation.

Disadvantage: identical launch command for every JAR, making differentiation difficult.

Solution 2 – Spring Boot loader.path

When the application is packaged as an executable JAR, the -jar launch disables -cp. Adding loader.path allows additional JARs to be loaded, but the Main-Class must be set to PropertiesLauncher in the Maven plugin configuration:

<build>
  <plugins>
    <plugin>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-maven-plugin</artifactId>
      <configuration>
        <includeSystemScope>false</includeSystemScope>
        <layout>ZIP</layout>
      </configuration>
      <executions>
        <execution>
          <goals>
            <goal>repackage</goal>
          </goals>
        </execution>
      </executions>
    </plugin>
  </plugins>
  <finalName>Test</finalName>
</build>

The layout can be JAR, WAR, ZIP, MODULE, or NONE, each affecting the launcher class.

Solution 3 – Custom ClassLoader

Invoke a custom class loader at a specific program point to load JARs from a designated directory. Example code:

// custom class loading
SpringApplication.run(SpringBootDemo.class, args);

Solution 4 – JVM Options

Modifying -Xbootclasspath/a or java.ext.dirs changes the system class loader but is unsafe and not recommended.

Scanning Loaded Classes into Spring

If business JARs share a common package prefix, configure scanBasePackages in @SpringBootApplication to include that prefix, e.g., com.demo. For packages that do not match, add the appropriate entries to META-INF/spring.factories for automatic configuration.

# Auto Configure
org.springframework.boot.env.EnvironmentPostProcessor=\
  com.baomidou.mybatisplus.autoconfigure.SafetyEncryptProcessor
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
  com.baomidou.mybatisplus.autoconfigure.IdentifierGeneratorAutoConfiguration,\
  com.baomidou.mybatisplus.autoconfigure.MybatisPlusLanguageDriverAutoConfiguration,\
  com.baomidou.mybatisplus.autoconfigure.MybatisPlusAutoConfiguration